1. Field of the Invention
The present invention generally relates to the use of fluid catalytic converter (FCC) units. More specifically, it relates to methods for stabilizing the operation of multiple-catalyst-employing FCC units by use of predetermined amounts and ratios of certain catalysts used therein.
2. Description of the Prior Art
FCC units carry out processes that are employed throughout the petroleum refining, chemical and petrochemical industries. These processes are frequently destabilized by changes (deliberate, as well as unavoidable changes) such as (1) variation in the character of the feedstock being supplied to such units, (2) selection of different products to be made by such units, (3) selection of different grades of a product being made by such units, and (4) imposition of more stringent legal requirements (e.g., lower air pollution levels) upon such units.
Those skilled in this art will appreciate that any increase in a FCC operator""s ability to anticipate the operation of, and/or more closely control, a process being carried out by a FCC unit will usually serve to minimize the use of, and hence the costs of, the very expensive catalyst consumed by that unit. The ability to anticipate the operation of such units also serves to reduce the complexities associated with the sometimes competing, and sometimes complimentary, effects caused by simultaneous use of several different catalysts species in such units. A FCC unit that is operating on a more stable basis also will generally tend to provide end products having more consistent quality attributes.
In order to better appreciate applicant""s methods for stabilizing the operation of FCC units, it is helpful to envision their general mode of operation. Typically, a bulk catalyst inventory of tons, indeed, even hundreds of tons, of catalyst flow (often at high velocities) through the fluidized beds, reaction zones, and regeneration zones that make up such units. Next, it should be appreciated that the total catalyst inventory circulating through such a unit is usually comprised of a host catalyst (that carries out the primary catalytic function of the unit) and several distinct types of catalyst additives (that carry out secondary catalytic and/or sorbent functions). That is to say that various additives are used to carry out certain xe2x80x9csecondaryxe2x80x9d functions that are, in some way or another, associated with the primary catalytic function. Preferably, the catalyst additives are distributed, to a high degree of homogeneity, in the host catalyst. Since the host catalyst carries out the primary catalytic function of a FCC unit (e.g., cracking a petroleum based feedstock), it usually comprises from about 80 to about 99 weight percent of the total catalyst inventory in such unit. Catalyst additives usually comprise the remaining 1.0 to 20.0 percent of their total catalyst inventory.
Each catalyst particle species (host catalyst or catalyst additive) introduced into a FCC unit will eventually disperse through the existing host catalyst/catalyst additive inventory and, at its own rate, be chemically deactivated, attrited, broken into smaller and smaller fragments that are eventually elutriated from the unit. The rate at which each catalyst particle species is chemically deactivated, attrited and elutriated is determined by the catalytic activity, hardness, durability, particle size and density characteristics of that particular catalyst particle species. Ideally, each different catalyst species will be introduced into the FCC unit in a manner and at a rate such that the overall host catalyst/catalyst additive system will settle down to some desired steady-state performance level as quickly as possible.
An FCC operator also would like to be able to respond, as quickly as possible to any changes (e.g., changes in quality or product distributions) that might arise so that the unit can be brought back to a steady-state mode of operation. Such responses usually need to be carried out while, to the fullest extent possible, maintaining one or more desired FCC unit performance levels and while maintaining, as much as possible, one or more xe2x80x9csecondaryxe2x80x9d operating characteristics. For example, a FCC unit used to refine petroleum may be called upon to convert a given petroleum feedstock primarily into a gasoline product of a given octane rating at a given rate of production while, simultaneously, holding the unit""s output of pollutants (e.g., SOx, NOx, CO, etc.) to certain legally prescribed levelsxe2x80x94regardless of changes in the character of the petroleum feedstock (e.g., regardless of an increase in the sulfur content of the feedstock petroleum). Other common technical or economic operating goals or characteristics that a petroleum refinery operator might wish to achieve might include (but, by no means be limited to) (1) better control of the relative proportions of various end products being made by the unit (e.g., obtaining prescribed C3-C4 product yields while limiting production of ethane, ethylene, methane and hydrogen), (2) low coke lay down rates (i.e., low rates of coke deposit on the FCC catalysts particles), and/or (3) achievement of economic goals (for example, obtaining the greater economic value, and hence profit, associated with gasoline products having higher research octane numbers).
The individual catalyst particles used in FCC units are usually comprised of a catalytically active component (e.g., a zeolite) and a generally xe2x80x9cinertxe2x80x9d matrix or binder material that serves primarily to hold particles of the catalytically active component (e.g., zeolite) in a catalyst/binder composite particle. This binder/catalyst format is generally used to make both host catalyst particles and catalyst additive particles. Depending on the nature of the catalytically active catalyst, the process being carried out, the severity of the temperature, pressure, particle velocity, etc. conditions in the FCC unit, economic considerations, and so forth, any given catalyst particle may have relatively small amounts (e.g., 1%) to relatively large amounts (e.g., 50%) of the catalytically active component embedded in a binder or matrix material such as alumina, silica, etc.
Hence, the xe2x80x9cratiosxe2x80x9d of any two or more catalysts used in a FCC process are often normalized to a common unit of comparison such as a common unit of weight (e.g., pounds). That is to say that the weight of a first type of active catalyst in a first particle species, is preferably compared to the weight of a different type of active catalyst in a second particle species. For example, if the first particle species and the second particle species weighed the same, but the first particle species were comprised of 10% by weight of active catalyst A and 90% by weight of binder material, and the second particle species were comprised of 50% by weight active catalyst B and 50% by weight binder material, then in order to get the same amount by weight of catalyst A and catalyst B for a xe2x80x9cper unit weightxe2x80x9d comparison, five times as much of the first catalyst species would be employed.
Next, it should be appreciated that in most cases, any given FCC function, operating characteristic or parameter is often regarded as being achieved through use of one particular catalyst speciesxe2x80x94even if this may not be literally true. That is to say that any given FCC function may be, and often is, influenced to some degree by other catalyst species that are placed in a FCC unit in order to carry out other, often entirely different, catalytic functions. By way of example only, a catalyst additive species that is used to reduce SOx emissions may also effect NOx emission, CO emissions, and perhaps even the primary catalytic function being carried out by the bulk catalyst. Indeed, such secondary effects are known to take place as a result of the use of (1) different catalyst additive species, (2) different bulk catalyst species and (3) a given bulk catalyst species used in conjunction with a given catalyst additive species.
Moreover, the complexities associated with the simultaneous use of several different catalyst species will often be greatly magnified in those cases where the chemical reaction(s) being carried out in a FCC unit is (are) themselves complex. For example, since petroleum is comprised of literally hundreds of different kinds of molecules, any catalytic reactions carried out with respect to petroleum will be inherently complex. Indeed, this would be the case if only one catalyst species were being employed to refine a petroleum feedstock. When multiple catalyst species are employed with respect to such a complex array of molecular types, the situation becomes all the more complex. And, against this background of complexities, it also should be understood that every individual FCC unit has its own unique operating characteristics and that this fact introduces yet another layer of complexities into any attempts to control its operation.
The FCC prior art has dealt with this array of complexities in various ways. For example, FCC operators, using their experience and judgment as to the appropriate amount(s) of catalyst(s) to use, may simply add catalyst(s) to a base level at which they have had good experiences in the past and then try to meet each immediate problem (such as a change in a factor affecting an operating parameter, for example, a change in the character of the petroleum feedstock) as it arises. This method of adding catalyst(s) often wastes large amounts of a given catalyst in order to meet each new situation as it arises.
Experience has also shown that it is difficult to achieve long term operating stability, and hence maximum profitability, by this method since most operators are usually more concerned with getting an xe2x80x9cupsetxe2x80x9d FCC unit back to some desired output level as quickly as possiblexe2x80x94rather than immediately concerning themselves with the optimum amounts of the catalyst needed to do this on a longer term basis. It also should be noted that, when such upsets occur, the all too human tendency of many FCC operators is to add much more of a given catalyst than is actually prescribed in order to correct the problem at hand as quickly as possible in order to get through their work shift with as few glaring problems as possible. Unfortunately, a high price is usually paid for these xe2x80x9coverdosingxe2x80x9d practices since catalysts in general, and catalyst additives in particular, are very expensive materials. Moreover, any given overdosing of one catalyst species often creates competing demands or problems that create a need for the use of another catalyst species which also may be overdosed and lead to a cascading series of upset conditions.
This situation was improved somewhat when the addition of catalyst(s), and especially catalyst additives, was (were) managed by programmed computerized control devices such as central control units (CPUs) that both continuously monitor and control the amounts of any given catalyst species added to a FCC unit. Ideally, a given amount of catalyst (e.g., a catalyst additive) is injected into a FCC unit at regular time intervals. These regular time intervals are usually established by prior testing of that particular FCC unit with respect to its use of a given catalyst species.
Each such catalyst addition can also be programmed and controlled by a CPU to respond to upset conditions as they arise. For example, the catalyst system disclosed in U.S. Pat. No. 5,389,236 (xe2x80x9cthe ""236 patentxe2x80x9d) teaches addition of a catalyst to a FCC unit either on a predetermined schedule or on an xe2x80x9cas neededxe2x80x9d basis that may be determined by a discrepancy between the amount of catalyst that xe2x80x9cshould have been addedxe2x80x9d and the amount that is actually added. In this particular system, any differences in these two amounts are determined by constantly weighing each catalyst hopper feeding the FCC unit.
The ""236 patent also teaches that the most reliable basis for determining the proper amount of catalyst to be added to a given FCC unit, under a given set of operating conditions, can be determined by prior tests of that particular FCC unit wherein the effects of a given catalyst species on a given operating parameter are established by injecting a known amount of that catalyst species into that unit and then noting its effect on a selected FCC parameter. In other words, this is the FCC art""s attempt at invoking one of the most venerable techniques known to science and engineering, namely: xe2x80x9cholding all other things equalxe2x80x9d while varying one factor (the injection of a specific catalyst species) and then noting the consequences of that catalyst injection on a given operating parameter.
In the case of testing FCC units, however, holding all other variables constant in order to test the effects produced by a given catalyst species has proven to be a difficult (and many would say xe2x80x9cimpossiblexe2x80x9d) task. Still, this method of establishing what effects will follow from the use of a given catalyst species, under the conditions created by a given FCC unit, is widely employed because, heretofore, there has been no better way of dealing with the complexities inherent in the operation of those FCC units that simultaneously employ multiple catalyst species.
Consequently, the effects of a given catalyst species on a given operating characteristic of a given FCC unit are tested by injecting a given amount of a single catalyst species into a given FCC unit on a one time, two time, three time, etc. basisxe2x80x94over a given time span. The effects of such injections on the operating parameter are then plotted in order to gain insights into how that particular catalyst species affects the operating parameter under consideration. This information is then used to create an operating curve based upon the effects of the given catalyst species with respect to the given operating parameter (e.g., SOx emissions). A similar test might be conducted with respect to that catalyst""s ability to affect some other operating parameter (e.g., NOx emissions). Such tests with respect to other parameters will produce other operating curves that a CPU may be programmed to follow. In effect, these operating curves are stored in the CPU""s memory and used as reference curves for future operation of the FCC unit""s catalyst addition system. Heretofore, this method of testing the ability of a single catalyst species to affect a given operating parameter has not produced particularly good resultsxe2x80x94and hence, have not produced particularly reliable operating curves upon which to base future operation of the FCC unit.
Applicants have found that much better insights into the operation of a given FCC unit with respect to various operating characteristics or parameters of that unit (e.g., C3-C4 yields, operating costs, SOx emissions, NOx emissions, CO emissions, CO combustion, gasoline sulfur reduction, olefin product make, olefin product molecular structures, vanadium reduction, etc.) can be achieved if the unit is subjected to injection tests wherein two or more catalysts species are tested more or less contemporaneously. Preferably, each of the two or more catalyst species being used in such test injections should have, in its own right, a detectable effect on the same operating parameter. For example, if each of two different catalyst species (e.g., catalyst species A and catalyst species B) each has an effect on, for example, SOx emission levels from the FCC unit, a more accurate and/or more useful operating curve can be obtained (relative to the accuracy and usefulness of two separate curves created by separate testing of each of the two different catalyst species). These more accurate and/or more useful operating curves are subsequently used to control the FCC unit during its subsequent xe2x80x9cnormalxe2x80x9d use (as opposed to its use in the initial testing procedures that produce such unit response curves).
Therefore, the testing methods described in this patent disclosure are based upon use of at least two (and preferably more) injections of at least two different proportions of multiple catalyst species in order to establish an operating curve with respect to a given operating characteristic or variable. The goal of applicant""s testing methods is to obtain an operating curve that is thereafter used to attain (or to maintain) a given performance in the FCC unit with respect to a given operating parameter (e.g., SOx production, NOx production, C3-C4 yields, CO emissions, CO combustion, gasoline sulfur reduction, olefin product make, olefin product molecular structures, vanadium reduction, operating cost, etc.) that is influenced by each of the two or more catalyst species that are to be employed in the FCC unit. Again, each of the catalyst species used in such tests should be used in an amount that is, in and of itself, capable of effecting the operating parameter being studied. It should also be noted that during subsequent operation of the FCC unit, changes in the operation of that unit can be affected by injection of either or both of the subject catalysts (e.g., by an injection of catalyst A, catalyst B or a mixture of catalyst A and catalyst B).
Those skilled in this art will appreciate that almost all catalysts introduced into a FCC eventually will become xe2x80x9cinactive.xe2x80x9d This inactivity results from the fact that the catalyst particles can no longer be regenerated (e.g., by burning off carbonaceous materials deposited on the surface of the catalyst particles), and/or because the catalyst particles are eventually physically destroyed by the harsh environments existing in FCC units. Depending on the nature of the specific catalysts species, such deactivation may take a few hours or several days. The length of the deactivation period for any given catalyst species to some degree, will be known by the prior experience of FCC operator and/or the catalyst manufacturer. For example, the deactivation period for a SOx catalysts may be 3 or 4 hours while that of a zeolite hydrocarbon cracking catalyst may be 3 or 4 days. Be the length of such deactivation periods as they may, the preferred method for conducting the tests of this patent disclosure is to make the first catalyst A:catalyst B injection and wait then for the catalyst A:catalyst B system to substantially (e.g., more than 60% deactivate) deactivate. After the longest-lived catalyst of the catalyst system (catalyst A or catalyst B) has substantially deactivated, the second test is carried out using a different catalyst A:catalyst B ratio. That is to say that, preferably, the second test will give the most accurate test results after the catalytic effects of the first test have substantially xe2x80x9cworn offxe2x80x9d i.e., less reliable (and hence less preferred) test results will be obtained in a second test if the FCC unit is still under the effects of the first test.
The testing and FCC unit stabilization and/or control methods of this patent disclosure may be expanded to include 3, 4, 5, etc. different catalyst species used in 3, 4, 5, etc. different relative proportions or ratios. Again, it is preferred that each such catalyst species be capable of influencing the same operating parameter, e.g., SOx emissions, NOx emissions, octane number, etc. For example, a three catalyst system, wherein each catalyst can effect a given operating parameter, could be based upon a first injection of a A:B+C catalyst system wherein equal amounts of catalysts B and C are used while the amount of catalyst A is systematically raised during 2, 3, 4, 5, etc. injections of the A:B+C catalyst system. Again, this test technique may be expanded to a A:B+C+D catalyst system, a A:B+C+D+E catalyst system and so on.
Employment of the methods of this patent disclosure normally will begin with the identification of a particular operating characteristic or parameter of an FCC unit that is to be studied, and thereafter influenced during normal operation of that unit. The study will be based upon injection of two or more different catalyst species into the unit, at two or more different times, using two or more relative proportions of the subject catalysts. The two or more different catalysts also should be injected into the unit in a time frame such that the presence of one catalyst will influence the operating parameter under study while the second, third, etc. catalyst is also influencing that same operating parameter. In the more preferred embodiments of this invention, the two (or more) catalysts that influence the same operating parameter will be introduced into the FCC unit more or less contemporaneously. The FCC unit may (or may not) be operating under a given operating parameter limitation at the time of these test injections. For example, such a unit may be operating under a legal limitation (e.g., a 700 ppm SOx emissions level) but may not be operating under a given technical or economic limitation such as maximization of yield of C3-C4 product to maximize profit.
In any case, during such tests, each catalyst A:catalyst B system (or A:B+C or A:B+C+D, etc. is catalyst system) should be introduced into the FCC unit in an operating characteristicxe2x80x94effecting amount and in at least two different relative proportions or ratios (e.g., 1A:1B, 2A:1B, 1A:2B, etc.). The effect or influence of those two or more catalyst ratio test injections on the operating parameter being studied (e.g., SOx emissions) are, in some appropriate manner, observed and/or recorded. This information is then used to establish a test response curve for the catalyst ratios versus the operating parameter that is influenced by those two (or more) catalysts.
During subsequent normal operation of the subject FCC unit, the previously selected operating parameter (e.g., SOx emissions) is monitored. If the operating parameter falls away from a value corresponding to a value associated with the test response curve, the FCC unit is returned to a mode of operation that comports with the mode of operation associated with the test response curve that was established by the initial catalyst injection tests. This return to a desired level of the operating parameter during normal operation of the FCC unit is accomplished by introducing at least one of the two or more catalysts into the FCC unit in an amount such that the catalyst A:catalyst B ratio (the catalyst A: catalyst B+catalyst C, etc. ratio) in the FCC unit is brought to a level that corresponds to, or is associated with, a value lying xe2x80x9conxe2x80x9d (or near i.e., within a given tolerance, e.g., xc2x110%) the test response curve that was established by the initial testing of the FCC unit.
The concept of a xe2x80x9ctest response curvexe2x80x9d also may be expanded to include a xe2x80x9ctest response envelopexe2x80x9d by use of certain hereinafter described methods. In either case, during subsequent operation of the FCC unit, various amounts of catalyst A, catalyst B, catalyst C, etc., or mixtures of various proportions of catalyst A, catalyst B, catalyst C, etc. can be introduced into the FCC unit in order to bring its operation unto correlation with the test response curve (or test response envelope). In some of the more preferred embodiments of this invention, at least one of the different catalyst species (A and B) will be introduced into the FCC unit to xe2x80x9ccorrectxe2x80x9d its operation with respect to the operating parameter in question. In other embodiments of this invention, two or more operating-parameter-effecting catalysts (A, B, C, D, etc.) will be introduced into the unit to make such corrections.
Thus, stated in patent claim terminology, the hereindescribed methods for controlling the operation of a multiple-catalyst-employing FCC unit, will comprise: (1) selecting an operating characteristic of the FCC unit that is affected by each of at least two different catalyst species: catalyst A and catalyst B; (2) injecting an operating characteristic-affecting amount of catalyst A and an operating characteristic-affecting amount of catalyst B into the FCC unit in a first catalyst A:catalyst B test ratio; (3) determining the effect of the first catalyst A:catalyst B test ratio on the operating characteristic in that FCC unit; (4) injecting an operating characteristic-affecting amount of catalyst A and an operating characteristic-affecting amount of catalyst B into the FCC unit in a second catalyst A:catalyst B test ratio that is not the same as the first catalyst A:catalyst B test ratio; (5) determining the effect of the second catalyst A:catalyst B test ratio on the selected operating characteristic of the FCC unit; (6) establishing a unit response curve (or unit response envelope) for the operating characteristic based upon the effect of the first catalyst A:catalyst B test ratio and the second catalyst A:catalyst B test ratio on said operating characteristic; (7) selecting a desired level of the operating characteristic that can be attained by use of a catalyst A:catalyst B system in that FCC unit during its subsequent normal operation (i.e., during xe2x80x9cnormalxe2x80x9d or xe2x80x9creal timexe2x80x9d production operations); (8) programming a unit response curve (or unit response envelope) based, at least in part, upon the unit response curve (or unit response envelope) established by use of the catalyst test ratios in a CPU unit that controls a catalyst addition system associated with the FCC unit, and (9) injecting at least one of the two catalysts (catalyst A and/or catalyst B) into the multiple-catalyst-employing FCC unit (during subsequent normal operations of said unit) in an amount that brings the active catalyst A:catalyst B ratio in the FCC unit to a level or value that correlates with some point on the unit response curve (or some place in the unit response envelope).
In some more preferred embodiments of this invention, the test response curve (or test response envelope) is established through use of more than two injections of a catalyst A:catalyst B (or catalyst A: catalyst B+catalyst C, etc.) system. More such tests tend to produce more accurate curves (or envelopes). Be that as it may, catalyst A and/or catalyst B, etc. are injected into the FCC unit during subsequent production operations in amounts such that the catalyst A:catalyst B ratio in said FCC unit is brought to a value that correlates to some desired point on the unit response curve (or unit response envelope) for the operating characteristic. The methods by which this can be done, and the advantages obtained by employing such methods, can be better understood by studying the following, more detailed, descriptions and examples of this invention.